1. Field of Invention
This invention concerns a metallization process in VLSI (Very Large Scale Integrated) and ULSI (Ultra Large Scale Integrated) device fabrication. More precisely, this invention concerns a method for forming passive barrier metallic layers on a patterned surface of a Si substrate using a magnetron sputtering technique.
2. Description of Related Art
The magnetron sputtering apparatus shown in FIG. 4 is a device previously known. That apparatus includes a vacuum chamber [1], a target [2], a permanent magnet assembly [3], a permanent magnet [5], a substrate holder [4], a substrate [6], and a DC power source [7]. The cathode assembly includes the target [2] and the permanent magnet assembly [3], and the anode assembly includes the substrate holder [4], the permanent magnet [5] and the substrate [6]. An electric field is created in the space defined between the target [2] and the substrate [6]. The electric field is created by applying a voltage to a target [2] by the DC power source [7] while a magnetic field perpendicular to the electric field is created by the permanent magnet assembly [3] and the permanent magnet [5]. The vacuum chamber [1] is evacuated by a vacuum pump system [18], and the pressure in the vacuum chamber [1] is maintained from several mTorr's to several tens of mTorr's by introducing process gas such as argon and the like through the gas supply system [19]. Plasma [8] is formed by a magnetron discharge in the space defined between the target [2] and substrate [6]. As a result, ions in the plasma [8] sputter the target [2]. The particles sputtered out of the target [2] are deposited on the surface of the substrate [6], and consequently, a thin film is formed. Furthermore, the permanent magnet assembly [3] is rotated around an axis perpendicular to the plane of the target [2] to improve uniform thickness distribution across the thin film deposited on the substrate [6].
Such a sputtering apparatus is available for, especially, fabrication of semiconductor devices. For example, a titanium film is formed with the use of a target made of titanium metal and argon as a process gas. As another example, in the sputtering apparatus, a titanium nitride film is formed with the use of a target made of titanium metal and either nitrogen gas or a mixed gas of nitrogen and argon.
A multilayer film can be formed on a substrate surface in an integrated module multi-chamber vacuum processing system, instead of the vacuum chamber [1]. In each chamber of such a system, a layer of each different kind is deposited, respectively. By this system, the film is improved in quality as well as in productivity. For example, semiconductor manufacturing equipment "ANELVA-1051 and 1052", sold by ANELVA Corp., is an integrated module multi-chamber vacuum processing system.
For deposition of a TiN film, it is preferable to use the so-called reactive sputtering technique to form a metallic compound film consisting of at least two elements. This reactive sputtering technique is based on the principles that (1) a consistent element of a target is ejected in the form of a particle into a plasma atmosphere by sputtering the target, (2) the consistent particle reacts with a particle composed of a consistent gas molecule in a plasma atmosphere, and (3) therefore a thin film consisting of at least two elements is formed. For example, for the deposition of a titanium nitride thin film, titanium particles are sputtered out of the titanium target. Reaction of the titanium particles with particles of nitrogen gas molecules results in deposition of a titanium nitride film. In this reactive sputtering technique, at least one element consisting of a deposited film is introduced in the form of gas so that the composition of the deposited film can be controlled by adjusting the flow rate or partial pressure of the consistent gas.
In the formation of TiN film, nitrogen-rich TiN film or titanium-rich TiN film can be formed by controlling the flow rate of nitrogen gas. For successive depositions of Ti and TiN film, a combination process of Ti film deposition without the use of nitrogen gas and TiN film deposition with the introduction of nitrogen gas enables successive applications without changing the film deposition process of the target.
FIG. 5 shows an example of a multilayer film used in the fabrication of a semiconductor device. Reference number 9 in FIG. 5 indicates a contact hole made in a BPSG (boro-phospho silicate glass) film [10] on the surface of a substrate [6]. The multilayer film is titanium film [17] with titanium nitride film [16] as the first layer, an aluminum silicon alloy film [12] as the second layer and silicon film [13] as the third layer, all of which are formed onto the contact hole.
To form this passive barrier metallic layer, Ti film [17] and TiN film [16] can be deposited in different vacuum chambers for each film deposition process. However, it is preferable from the viewpoint of productivity, that the double-layered barrier metallic layer [11] of Ti/TiN film is formed by changing the composition of a mixed process gas in the same vacuum chamber [1]. In that case, it is difficult to attain uniform thickness distribution across both Ti and TiN film on the whole surface of the substrate [6]. The uniformity becomes more important, particularly as the diameter of a substrate becomes larger. Uniform thickness distribution on the whole surface of the substrate is required in a semiconductor wafer that is eight inches in diameter, which is now predominantly used in the production process of semiconductor devices.
In general, a practical thickness distribution (.+-.3-.+-.5%) can be attained in both a Ti film formed with argon gas as the process gas and with a TiN film formed with the use of nitrogen gas as the process gas in the same chamber [1] provided with a titanium metal, wherein the distance between the target [2] and substrate [6] (which is called the T/S distance) is kept constant. However, as is mentioned below, that TiN film does not have appropriate properties as the barrier metallic layer. However, if a mixed gas of argon and nitrogen gases (with a composition ratio of 1:1) is used, a barrier metallic layer for practical use can be formed. However, successive deposition processes in the same chamber have not been successful so far because it has been necessary to set the T/S distance longer in the TiN film deposition process than in the Ti film deposition process to attain practical thickness distribution (within .+-.5%).
It has been pointed out earlier that the dependence of thickness distribution on the whole substrate surface on a composition of the mixed process gas arises from a subtly different degree in which the target surface topography is changing. Such a change in thickness distribution is shown in FIG. 6. The abscissa represents the T/S ratio which is derived by dividing the distance between the target and substrate (T/S distance) by the target diameter. The ordinate represents the thickness distribution on the whole surface of the substrate. When the target diameter is 300 mm, the T/S ratios of 1/5 and 1/3 at the abscissa correspond to the T/S distances of 60 mm and 100 mm, respectively,
When the T/S ratio is equal to 1/5 , the thickness distribution across a Ti thin film is attainable below .+-.5% but the thickness distribution across a TiN thin film exceeds .+-.10% (TiN thin film formed with a composition ratio of argon and nitrogen gas (Ar/N.sub.2) of 1/1 ). When the T/S ratio is equal to 1/3 , the thickness distribution across a Ti thin film, however, exceeds .+-.10%. However, the thickness distribution across a TiN thin film (Ar/N.sub.2 =1/1 ) is below .+-.5%. Thus, until now no system has attained the same uniformity of thickness distribution of these films simultaneously at the same T/S ratio. When the T/S ratio is approximately equal to 1/4 (T/S distance is equal to about 75 mm, with the target diameter being 300 mm), each thickness distribution is the same. However, the thickness distribution is over .+-.5% for each layer. As a result, the total thickness distribution of the Ti/TiN double-layered film is over .+-.5%, which is of no practical use.
The dotted curve represents a TiN thin film formed with a composition ratio Ar/N.sub.2 of 0/1. The distribution characteristic thereof is similar to that of Ti film, so that a T/S ratio of 1/5 may yield a uniform thickness distribution. However, the barrier metallic layer composed of TiN film formed in an atmosphere of pure nitrogen gas (Ar/N.sub.2 =0/1) is of no practical use for the barrier metallic layer in respect of (a) film quality and (b) electrical properties as compared to that formed in an atmosphere of a mixed gas of argon and nitrogen gases. Regarding the film quality, the barrier metallic layer composed of the TiN film formed with Ar/N.sub.2 of 0/1 is apt to crack easily. Through the cracks, constituent atoms of the layers separated by the barrier metallic layer diffuse, and therefore, constituent atoms mix with each other.
Regarding the electrical properties, for example, a barrier metal resistivity in the range of 100 to 250 .mu..OMEGA..multidot.cm is required for holding the electric conductivity of an Al layer deposited as an electrode metal wiring. However, the resistivity of the barrier metallic layer composed of the TiN film deposited with Ar/N.sub.2 of 0/1 is beyond the required range. Therefore, it is necessary for these thin films to be formed in an atmosphere of the mixed Ar and N.sub.2 gases.